Method and plant for tangential filtration of a viscous liquid

ABSTRACT

The invention relates to a method of tangential filtration of a viscous fluid or liquid in which a supercritical substance is dissolved in a supercritical state in this viscous fluid or liquid so as to lower the viscosity. The invention also relates to the installation for the implementation of the method. The invention is applicable particularly to the filtration of heat sensitive organic liquids and used motor oils.

This application is a 371 of PCT/FR98/00746, filed on Apr. 14, 1998,which is based on application FR/970619, filed on Apr. 15, 1997.

DESCRIPTION

The invention relates to a method of tangential filtration of a viscousfluid or liquid in which a third substance is dissolved in asupercritical state in this viscous fluid or liquid so as to lower theviscosity.

The invention also relates to the installation for the implementation ofthe method.

The invention is applicable particularly to the filtration of heatsensitive organic liquids and used motor oils.

The technological field of the invention is the filtration, moreprecisely the filtration of viscous fluids or liquids, and in particularthe tangential filtration of these viscous fluids or liquids.

Filtration in the traditional sense of the word is head-on filtration inwhich a flow of liquid meets a porous obstacle perpendicular to its pathwhich retains all the particles greater than the size of the pores. Theretained particles, the size of which is traditionally of the order offrom a millimeter to a micrometer, then form a cake which, in its turncontributes to the performance of the filtration. The particles whichare retained are smaller and smaller while the flow of filtrate reduces,this being the phenomenon of clogging up. The filtration conditionsnever reach a quasi-stationary state.

In tangential filtration, these disadvantages are done away with, sincethe fluid is carried in a dynamic fashion and the main flow of liquidruns parallel to the filtering surface which prevents it clogging up.

Because of this and provided that the operating conditions are wellchosen, the performance of the filtration in relation to retention andto flow can be considered to be constant over a sufficiently largeperiod of time, in other words the conditions are quasi-stationary.

In the case of viscous fluids, this filtration is hindered by therheological properties of these compounds.

In effect, the friction forces induced by the flow of a fluid through aporous body are very high in conformity with POISEUILLE's Law since theflow rate of an uncompressible liquid is in inverse proportion to theviscosity.

In tangential filtration, where the fluid, as has been seen above, iscarried in a dynamic fashion in order to avoid clogging up the filteringsurface, this disadvantage is even more important since the viscous dragthrough the porous body is added to the frictional forces due to theturbulent flow in the piping.

This is one of the reasons why the traditional methods of tangentialfiltration, such as, for example micro-filtration, ultra-filtration ornano-filtration essentially relate to aqueous liquids, the viscosity ofwhich is close to that of pure water, namely 1 mPa·s at 20° C.

With regard to organic liquids, their viscosity, when they are not puresolvents, can be very high, which makes their filtration impossible.This is notably the case for mineral oils that arise from petroleumfractions or vegetable or animal oils.

Hence various solutions have been proposed in the literature so as tooptimize or simply to make the filtration of viscous fluids possible, inparticular by tangential filtration.

The first solution consists of increasing the temperature of the fluidor liquid to be filtered so as to lower the viscosity.

Hence the document EP-A-0 041 013 describes a method known under thename of the REGELUB® method, in which used motor oils are treated, witha view to recycling them, at a temperature of from 250 to 300° C. tolower their kinematic viscosity from 150 cSt to 1 to 2 cSt. The usedoils, previously decanted to remove the water and distilled to removethe diesel oil or gasoline, are then filtered on ceramic mineralmembranes made of α alumina or on carbon based membranes, meaning thattwo batches are obtained: a first batch representing 10% of the initialvolume where the contaminants are concentrated by a factor of 10, and asecond purified batch, representing 90% of the initial volume, ofcommercial quality that simply requires a discoloration and anadjustment of the concentration of the additives.

The contaminants are essentially solid particles in the form ofsediments, sulfur containing compounds, metals stemming from wear in theengines such as iron, lead stemming from the fuel etc., as well ascalcium, magnesium, phosphorus etc.

The essential disadvantages of this method are:

the very high operating temperature which necessitates precautionsbecause of the dangers of the inflammability of the treated product; and

the mechanical stresses applied to the ceramic membranes. These stressesstem from the differential expansions between materials, caused by theheat during the start-up and the shut-down of the equipment.

At the present time and for techno-economic reasons, the industryprefers the sulfuric acid process to this method, despite its negativeenvironmental impact, notably due to the large amount of solid waste inthe form of sludges.

More generally, the solution that consists of heating the liquids to befiltered in order to reduce their viscosity cannot be used for heatsensitive products which would be degraded by the heating, and, on theother hand, it is restricted by the technology of “standard range”equipment. Because of this, temperatures of from 120 to 150° C. are notexceeded, which only causes a limited decrease in the viscosity.

A second solution that allows the filtration of viscous fluids consistsof adding at atmospheric pressure, a low viscosity solvent or “thirdsubstance”, soluble in the product to be filtered, in order to lower theviscosity.

For example, as described in the document FR-A-2 453 211, hexane can beadded to a motor oil.

In this way, the viscosity of the used oil will be reduced by a factorof 2 to 3 which allows a real improvement in tangential filtration equalto the ratio of the viscosities divided by the volumes of each of theconstituents. Therefore it is necessary for the viscosity to be stronglyreduced on adding small quantities of a third substance. Put anotherway, there is interest in adding a third substance of the lowestpossible viscosity.

The main disadvantage of such a method is to have to separate the twomiscible phases in an extra process step. For example, distillationwould be used to separate hexane from the oil phase. In addition to itrepresenting an additional step, this operation is costly in energywhich makes the method rather uncompetitive.

A third solution which makes the tangential filtration of viscousliquids possible consists of increasing the parietal stress by thefiltration of a two phase, gas/liquid mixture.

In this case, the working pressure is of the same order as fortraditional ultra-filtration since it reaches a maximum of 5 to 6 bars.This can qualify as atmospheric filtration since the permeate is atatmospheric pressure.

The gas is not dissolved in the liquid phase but is injected inco-current with the liquid, generally vertically. Because of thedifference in the density of the two phases, the bubbles of gas willhave an ascending speed greater than that of the liquid flow. They willcreate a “blockage” and the steric volume it occupies will destabilizethe polarization layer by lamination. In absolute terms, the velocitygradient will become very high locally, causing a significant increasein the flow of the permeate. This method, on a laboratory scale, hasalso been described in the document by M. MERCIER and C.DELORME—“Influence d'un écoulement diphasique gaz/liquide sur lesperformances de la filtration tangential”—Colloque UniversitéIndustrie—Toulouse 13.06.96, and is only applicable when the product tobe filtered does not foam.

A third method, which consists of using a “third substance” underpressure to treat hydrocarbons is described, for example, in thedocument FR-A-2 598 717 in which hydrocarbon oils are treated with asolvent to remove bituminous materials and to form a light oily phaseand a heavy bituminous phase. The solvent is recovered from the oilyphase by subjecting it to super-critical conditions for the solvent soas to form a new light phase enriched in solvent and a new heavy phase.This new light phase, very low in viscosity, is subjected to tangentialfiltration on a porous mineral membrane.

This method is getting close to being a liquid/liquid extraction, sincethe component in the liquid or super-critical state constitutes aseparate phase, the action of which is to extract the light extractablecomponents and/or, in parallel to this, cause a separation and aprecipitation of the heavy components.

Furthermore, it is not the heavy, viscous, liquid phase that is beingtreated by tangential filtration but the light phase which is a solutionhaving a very low viscosity, of the order of that of pure organicsolvents in the liquid or super-critical state, being between 0.1 and 1mPa·s.

Finally, the purpose of the filtration in this method is not tofractionate components of different sizes dispersed in a solvent, butrather to recover the pure solvent with a view to recycling it.

In addition, it is known that, in the absence of gas, viscosityincreases with pressure. This is particularly the case with compressiblepolymers, such as silicone oils.

On the other hand, if the polymer is in contact with a gas, the pressurewill dissolve the gas in the liquid phase. In this case, as studied byM. Daneshvar, S. Kim and E. Gulari in the document “High Pressure PhaseEquilibria of Polyethylene Glycols/Carbon Dioxide Systems” J. Phys.Chem. 94 (5) p.2124-2128, 1990, the solubility, for example of carbondioxide, reduces with temperature and increases with pressure while theviscosity reduces with the quantity of dissolved gas.

This principle has been exploited, but only for the head-on filtrationof extremely viscous products such as plastics in the molten state, asdescribed in document JP-A-87 206 471.

Similarly, document EP-A-0356 815 relates to the treatment ofpolycarbonates, polyester carbonates and aromatic polyesters which aremelted and then treated with a super-critical gas so as to reduce theviscosity, which makes their purification possible by head-on filtrationon fine filters, for example, on stainless steel filtration candles. Theinitial viscosities of the molten polymers are very high, for example,of the order of 900 Pa·s and are brought down to final viscosities ofabout 35 Pa·s by treatment with carbon dioxide gas at 250 bars and 257°C.

Such viscosities would certainly not permit tangential filtration ofthese fluids. In effect, as has already been mentioned, in tangentialfiltration, there is the absolute requirement to control a continuousprocess, notably by tangentially transporting a flow of permeate to thefiltering layer. Because of this, it is essential that the liquids mustbe less viscous, with a maximum viscosity of the order of 1 Pa·s.

It emerges from the work which has gone before that until now there hasnot been a tangential filtration method that allows viscous fluids orliquids to be treated in a satisfactory manner.

Therefore, there exists a need for a filtration method that has all theadvantages of tangential filtration and permits the filtration ofviscous fluids and liquids, in particular liquids which are sensitive toheat.

That is to say, there exists a need for a method for the tangentialfiltration of viscous fluids or liquids that operates preferably atmoderate temperatures.

The objective of this invention is to provide a method for thefiltration of a viscous fluid or liquid which does not have thedisadvantages of the prior art and which enables the problems met within these methods to be overcome.

The objective of this invention is also to provide a method for thefiltration of a viscous liquid which responds among other things to therequirements mentioned above.

This objective and others have been achieved, conforming to theinvention, by a method for the filtration of a viscous fluid or liquidhaving heavy and light components, and having an initial viscosity,under ordinary conditions of from 0.01 Pa·s to 1 Pa·s, characterized inthat, under the effect of pressure, a third substance in thesuper-critical state is dissolved in the viscous liquid, this thirdsubstance having a viscosity very much less than that of said viscousliquid, meaning that a single phase liquid solution is obtained whoseviscosity is reduced in comparison with the initial viscosity of thepure viscous liquid, said single phase liquid solution being treatedunder pressure by tangential filtration, to give, on the one hand aretained material comprising the third substance and the heavy componentand on the other hand a permeate comprising the third substance and thelight components.

In addition, it should be made clear that:

the heavy components without the third substance form the residue

the light components without the third substance form the filtrate

generally, part of the retained material is separated (diverted) anddischarged, generally on a continuous basis, this part being called the“concentrate”. Another part of the retained material, which keeps thename “retained material” is generally returned to the start of themethod downstream from the filtration.

The method according to the invention, due to the lowering of theviscosity obtained by dissolution of the third substance in the viscousfluid and, in general, without a large increase in temperature, permitstangential filtration of all types of viscous fluids and liquids.

The viscous fluids and liquids treated by the method according to theinvention generally have an initial viscosity, under ordinaryconditions, (that is to say, atmospheric pressure and 25° C.) of 10⁻²Pa·s to 1 Pa·s.

The viscous liquid generally referred to as the initial liquid or “firstsubstance”, contains, for example, the particles and/or molecules to beseparated, that is to say the “second substance”.

The field of application of the method according to the invention istherefore in the range of medium to high viscosities, as opposed to thevery high viscosities met with in methods such as that described indocument EP-A-0 356 815.

According to the invention, the third substance can be chosen from amongcompounds that are gaseous under ordinary conditions of temperature andpressure (1 bar, 25° C.) and are non-reactive to the initial viscousliquid.

The third substance can also be chosen from among compounds which areliquid under ordinary conditions of temperature and pressure and arenon-reactive to the initial viscous liquid.

In the case where the third substance is a liquid, the operatingconditions for the method must lead to it being in the super-criticalstate, in which it is of very low viscosity.

According to the invention, the viscosity of the third substance mustpreferably be very much lower than that of the viscous liquid, that isto say it must preferably be from 10⁻⁵ to 10⁻³ Pa·s, that is to say from100 times to 100 000 times lower than that of the viscous liquid.

The third substance is generally a pure compound but it can also beformed from a mixture of two or more pure components, each having thirdsubstance properties.

The single phase liquid solution (also called the “liquid phase”)obtained, also has a reduced viscosity which is generally of the orderof about a tenth to about a hundredth of the initial viscosity of theviscous liquid. Therefore the viscosity of the single phase solutionwill generally be 10⁻³ Pa·s. This reduction of the viscosity, due to thedissolution of the third substance leads to a very great improvement inthe permeability for the filtrate compared with filtration without athird substance at the same temperature.

The method according to the invention, operates advantageously atmoderate filtration temperatures, for example from 20 to 200° C.,preferably from 40 to 150° C., more preferably from 40 to 80° C.

Hence, in the case of heat sensitive products, the temperature of themethod can be held within limits compatible with the absence of chemicaldegradation. Such temperatures are significantly less than thetemperatures in the methods of the prior art. The operating conditionsfor the method according to the invention are therefore harmless for allviscous organic liquids, even the most heat sensitive. The methodaccording to the invention, for the first time permits tangentialfiltration of heat sensitive liquids and fluids which until now it hadbeen impossible to filter by traditional tangential filtration, that isto say filtration at atmospheric pressure. In the same way, the limitedtemperature allows inflammable liquids to be filtered under betterconditions of safety, without having recourse to complicated andexpensive equipment.

The range of moderate temperatures that are advantageously used by themethod do not impose great restrictions on equipment and is compatiblewith installations constructed using the usual materials without itbeing necessary to have recourse to special and costly materials.

The method according to the invention, due in particular to the lowviscosity of the liquid phase obtained by dissolution of the thirdsubstance, the moderate temperature used and the reduced circulationspeed used enables a significant improvement to be made in the energyefficiency of the method compared with the methods of the prior art.

By way of example, the speeds of circulation in the method of theinvention are from 0.5 to 10 m/s, preferably from 1 to 5 m/s and morepreferably from 1 to 4 m/s. These speeds are relatively lower comparedwith the methods of the prior art.

The pressures used in the method of the invention are, in all casesgreater than the usual pressures for tangential filtration methods suchas the atmospheric micro-filtration or ultra-filtration methods.

The working pressure obviously depends on the viscous fluid and on thethird substance used. It will generally be from 30 to 500 bars,preferably from 50 to 300 bars and more preferably from 100 to 500 bars.For each viscous fluid, there exists an optimum pressure which onlypermits the dissolution of the quantity necessary for the lowering ofthe viscosity without at the same time diluting the liquid phase toomuch.

It should be noted that the temperature and pressure conditions can bechosen to be sub-critical in relation to the third substance, orpreferably super-critical in relation to the third substance.

Pressurization in the method can be provided by the pressure of thethird substance present in excess in the gaseous or super-critical statewhich is supernatant above the liquid phase.

However the pressurization can also be provided by a neutral gas such ashelium, or by a gas having the characteristics of a neutral gas such asnitrogen, the partial pressure of which in the liquid phase can beneglected. It is desirable to operate in that way when the densitydifference between the third substance and the liquid phase becomes toolittle, for example at 400 kg/m³, since, in this case, the circulationof the liquid phase can lead to an uncontrolled two phase mixture in theseparation device such as a membrane.

The tangential filtration is applicable to the separation of anyparticles or molecules contained in the initial viscous liquid or fluid,whatever their size, and it can therefore be used for micro-filtration,ultra-filtration or nano-filtration.

The pressure used during the operation of tangential filtration ortransmembrane pressure depends on the technology used. It will generallybe from 1 to 6 bars for micro-filtration or ultra-filtration and from 5to 50 bars for nano-filtration.

According to the invention, the permeate—that is to say the fraction ofthe liquid phase which passes through the filtration device such as amembrane, and which is formed by the single phase mixture of the viscousliquid plus the third substance from which the “heavy” molecularcomponents and/or particles retained by the membrane are missing and inwhich the “light” compounds are to be found—and the separated (diverted)concentrate from the retained material—the retained material being thefraction of the liquid phase which is retained by the filtration devicesuch as a membrane, and which is formed by the single phase mixture ofthe viscous liquid plus the third substance in which the “heavy”components retained by the membrane—are advantageously treated byreducing the pressure (depressurization) meaning that the permeate isseparated into a filtrate comprising “light” components and the thirdsubstance and the concentrate is separated into a residue including“heavy” compounds and the third substance.

By lowering the pressure, it is understood that the pressure that ingeneral was from 30 to 500 bars at the start is in general reduced tobetween 1 and 5 bars.

Advantageously, according to the invention, the depressurization of thepermeate and/or the concentrate can be carried out in several steps, forexample, from 2 to 4 steps, each operating at pressures for example from500 to 300 bars, then from 150 to 50 bars, then from 50 to 1 bar,depending on whether one wishes to recover fractions that are more orless rich in the lighter components. These will be found in the main inthe later steps of the depressurization.

The third substance arising from the separation of the permeate and/orthat from the concentrate is recycled to the start of the method whichcontributes favorably to the mass and energy balance sheet for themethod.

Preferably, in order to permit this recycling, the pressure at the endof the depressurization must be equal to the pressure at which the thirdsubstance is supplied.

The method, prior to the tangential filtration can advantageouslyinclude an additional decantation/separation step. In effect the initialbringing into contact of the viscous liquid and the third substance canlead to separation of heavy components which are soluble in the pureviscous liquid but insoluble in the liquid phase. This phenomenon isknown under the name of the “anti-solvent effect”. The interest of sucha preliminary supplementary step is to anticipate the separation inrelation to a single tangential filtration step and to allow an increasein the flow of the permeate. The separation of the heavy componentswhich are difficult to separate, is obtained very easily by a simpleaddition of a third substance thanks to this advantageous step of themethod according to the invention.

Finally, the method according to the invention can be implemented in acontinuous manner or in a non-continuous manner, in other wordsbatch-wise. In this latter case, the viscous liquid and the thirdsubstance must be supplied in one go, before the tangential filtrationstage. This is of particular interest for products with high addedvalue, available in small quantities and for which continuous operationis not suitable.

Among the compounds gaseous at ordinary temperatures, that are suitableas third substances, one could mention by way of example: carbondioxide, helium, nitrogen, nitrogen monoxide, sulfur hexafluoride,gaseous alkanes with from 1 to 5 carbon atoms: methane, ethane, propane,n-butane, pentane, neo-pentane, gaseous alkenes having from 2 to 4carbon atoms: ethylene, propylene, butene; gaseous alkynes: acetylene,propyne and butyne-1; gaseous dienes such as propadiene; gaseousfluorinated hydrocarbons, gaseous chlorinated and/or fluorinatedhydrocarbons, for example the chlorofluorocarbons called “Freon®” andothers known as CFCs or HCFCs etc.

Among the compounds liquid at ordinary temperatures that are suitable asthird substances, one could mention by way of example: alkanes from 5 to20 C., such as n-pentane, iso-pentane, hexane, heptane, octane, liquidalkenes with from 5 to 20 C. atoms, liquid alkynes with from 4 to 20 C.atoms, alcohols such as methanol, ethanol, ketones such as acetone,ethers, esters, liquid chlorinated and/or fluorinated hydrocarbons etc.

Usually the third substance is in excess with respect to the liquidphase, that is to say it is present at a quantity of from 1 to 10 timesthe quantity of liquid phase.

In this case, the thermodynamic equilibrium is considered to be achievedand the viscosity is a minimum.

However it is also possible to dissolve a quantity of the thirdsubstance that is less than the maximum solubility under the operatingconditions aimed for: this quantity being for example from 0.1 to 1.0times the quantity of liquid phase. Since the amount dissolved is notthe maximum, the viscosity obtained will be greater than the equilibriumviscosity, but in the case where one wishes to avoid any risk ofseparation of heavy components, the “anti-solvent” effect is reduced inthis way.

The viscous fluids or liquids which can be treated by the method of theinvention are, for example, organic or aqueous fluids or liquids, inparticular organic, heat sensitive fluids or liquids containing heatsensitive products such as vegetable oils or animal oils, body fluids,various food products, liquids arising from agriculture, and aqueousphases containing proteins. Among the vegetable oils, one could mentionfor example, olive oil, sunflower oil, oregano oil, argan oil.

Among the animal oils, one could mention for example the fish oils suchas poor cod oil, sardine oil and cod liver oil.

The viscous fluid or liquid can also be chosen from among the mineraloils, for example, those arising from petroleum fractions, siliconeoils, industrial oils or fluids, motor oils, cutting oils, wire drawingoils, petroleum oils such as the residues from the distillation of rawpetroleum, used oils from all sources such as used motor oils, usedindustrial oils etc., liquids or fluids from industrial processes loadedwith particles and/or heavy components, for example catalyst particles,molten polymers such as polyethylene glycols (PEGs) etc.

Hence the method of the invention may be applied to the regeneration ofused oils with a view to their regeneration and their recycling.

Today, the major part of recyclable used oils are the black oils, madeup in part by industrial oils, for example wire drawing oils, or moreimportantly by motor oils. Their recycling which constitutes close to60% of the total, or 268 000 tons in 1992 for France, poses problems ofresidual pollution whether this is by incineration in cement producingplants because of the gaseous discharges or by the sulfuric acid processand to the earth because of the solid wastes in the form of sludges.

The method for the treatment of used oils called the REGELUB® method,described above, has, as we have seen the major disadvantage of having aworking temperature that is too high and of the order of 300° C.

The method, according to the invention, enables problems associated withthis method to be alleviated while obtaining viscosities of the sameorder of magnitude, namely between 1 and 3 mPa·s.

The single phase liquid phase obtained by dissolving a third substancesuch as CO₂ and made up of the raw oil and the gas is then treated onmembranes made of ceramic with a cut-off threshold compatible with thecomponents to be retained, in general within the range ofmicro-filtration or of ultra-filtration.

An extra advantage of the method is the separation of tars which are tobe found in these oils following the combustion of fuel during operationof the engine. This separation is directly obtained by a simple additionof CO₂, causing an anti-solvent effect.

Furthermore, the pollutants generated during operation of the engine,which are notably metals, are concentrated in a residual phase that iseasy to treat and that produces waste that is ultimately less than thatfrom the sulfuric acid process.

The filtrate obtained, that usually represents 90% or more of theinitial quantity of viscous fluid, has the same characteristics as thatobtained through the REGELUB® process.

The method of the invention can also find an application in thepetrochemical industry for the removal of asphaltenes and catalystfines, contained for example in a petroleum fraction.

In effect, it is known that the raw residues obtained after separationof the light fractions generally by distillation, can be used to formproducts of value by catalytic cracking. The aim of this operation is tohydrogenate and then to “crack” the heavy molecules so as to reduce themolecular mass and to make them more suitable for combustion. Thisallows them to be used as a fuel in the same way as the initial lightfractions.

This operation is usually carried out after dissolution of the residuesin a low boiling point solvent. An alkane can be used under pressure,for example, propane as in the ROSE process (“Residuum Oil SupercriticalExtraction”).

There are two disadvantages associated with the operation of catalyticcracking:

poisoning of the catalyst by metal compounds, even when these arepresent in trace quantities. They are essentially compounds formed bynickel, molybdenum, vanadium complexed by asphaltenes,

the presence of catalyst fines which must be separated from thepetroleum fraction and if possible recycled.

In the first case, the use of the method according to the inventionallows one to separate the asphaltenes containing the metals. Thisoperation takes place before the catalysis.

In the second case, the method according to the invention allows one toseparate the catalyst fines. This operation takes place after thecatalytic step.

The method according to the invention applied to the ROSE process or tosimilar processes is all the more interesting since a third substance inthe supercritical state, in this case propane, is already available.

The method according to the invention is applicable in a particularlyadvantageous manner to the tangential filtration of heat sensitiveliquids or fluids or containing heat sensitive products, in particularliquids or fluids of animal, vegetable origin, food products etc.

In this application, the temperature is held within the limits ofchemical or biological compatibility of the product.

A particularly preferred application is the treatment, at a temperature,for example of 40° C., of fish oils such as poor cod oil, which onewishes to enrich the triglyceride fraction in C20 and C22polyunsaturated fatty acids (eicosapenta-enoic acid:EPA anddocosahexa-enoic acid:DHA). These fatty acids have a beneficial effectfor the lowering of VLDL (Very Low Density Lipoproteins) levels, andreduces platelet aggregation, which justifies interest in them for theprevention of coronary illness.

Carbon dioxide is clearly indicated for this application, taking intoaccount its innocuous nature in relation to foodstuffs. The membraneused will preferably be a nano-filtration membrane the cutoff thresholdof which is close to 800 g/mole. The light triglycerides, statisticallylow in C20 and C22, pass through the membrane, while the heavy fraction,rich in C20 and C22 is held back in the retained material/concentrate.The phospholipids, insoluble in CO₂, separate and can be removedseparately.

The method according to the invention is also applicable in aparticularly advantageous manner to the treatment of aqueous phasesnotably containing proteins such as food proteins, for which, in asurprising way, an effect is observed, referred to as an “anti-clogging”effect, which can be explained in the following manner.

The dissolution, for example of CO₂, under the effect of the pressurecauses the formation of carbonic acid. This is expressed by an increasein acidity, that is to say by a reduction in pH. For a solubility of theorder of 60 g per liter, obtained at P: 300 bars and T: 60° C., the pHis of the order of 3.5.

The resultant acidity has the consequence of modifying the stericconformation of the proteins. In effect, the reduction in the pH causesa charge effect which tends to move the proteins away from theiriso-electric point (that is to say the point of zero charge, reached ata pH of around 4 to 5 for most foodstuff proteins). The modification ofthe pH also tends to move the ceramic that forms the membrane (or anyother inorganic or organic material) away from its iso-electric point(reached at pH 5.5 in the case of TiO₂). The final and unexpected resultis that the proteins mutually repel one another, thereby keeping theirconformation in a cushion, and are also repelled by the membrane. Theend result is that, the rate of retention is improved while at the sametime clogging up is reduced.

Finally, the method according to the invention can also be applied whena nano-filtration is being carried out, with the reduction of the degreeof polydispersity of the polyethylene glycols (PEG) or of any otherpolydisperse polymer.

Another subject of the invention is an installation for theimplementation of the method described above.

This installation is characterized in that it comprises means forsupplying a viscous liquid and a third substance, means of dissolvingthe third substance in the viscous liquid, means of tangentialfiltration, means of transferring and circulating the single phaseliquid in order to pass said solution into said means of tangentialfiltration.

The installation for implementing the method according to the inventionadditionally and advantageously includes means to lower the pressure ofconcentrate and permeate, and to separate the permeate from the filtrateand from the third substance and to separate the concentrate from theresidue and from the third substance.

The installation also includes means of recycling to the input to theinstallation, the separated third substance, arising from the separationfrom the permeate and from the concentrate.

Finally, the installation comprises regulating means for regulating thesupply and the tapping off.

This regulation is preferably coupled to the flow rate of the permeate.

Other characteristics and advantages of the invention will better becomeapparent on reading the description which will follow, which is givenpurely for illustrative purposes and is non restrictive and which makesreference to the appended drawings on which:

FIG. 1 shows a diagrammatic section view of an example of aninstallation for implementing the method of tangential filtration underpressure of viscous liquids with the addition of a third substanceaccording to the invention.

FIG. 2 is a graph which shows the density of the filtrate flow J inkg/h/m² in relation to the transmembrane pressure ΔP in bars for thepolyethylene glycol 400 to 60° C. on a nano-filtration membrane.

The curves A, B, C, D, E, F correspond respectively to partial pressuresof CO₂ of 0, 30, 60, 90, 120 and 150 bars.

FIG. 3 is a graph which represents the density of the filtrate flow J inkg/h/m² in relation to the transmembrane pressure ΔP in bars for thepolyethylene glycol 400 to 60° C. on a single channel ultra-filtrationmembrane.

The curves A, B, C, D, E, F correspond respectively to partial pressuresof CO₂ of 0, 52, 61, 91, 121 and 151 bars.

FIG. 4 is a graph analogous to that in FIG. 3 for a temperature of 40°C.

The curves A, B, C, D, correspond respectively to partial pressures ofCO₂ of 0, 101, 122 and 152 bars.

FIG. 5 is a graph analogous to that in FIG. 4 for a temperature of 75°C.

The curves A, B, C, D, E correspond respectively to partial pressures ofCO₂ of 0, 53, 103, 122 and 153 bars.

FIG. 6 is a graph which shows the density of the filtrate flow J inkg/h/m² in relation to the transmembrane pressure ΔP in bars for new andused motor oil at 75° C. on a multi-channel ultra-filtration membrane.

The curves A, B, C, D, E, F, G correspond respectively to partialpressures of CO₂ of 0, 51, 76, 101, 110, 120 and 151 bars and give theresults obtained for the new oil.

Curve H corresponds to a partial pressure of CO₂ of 101 bars and givesthe results obtained for used oil.

FIG. 7 is a graph representing the change in the permeability to theused oil J/ΔP in kg/(h.m².bar) as a function of time in hours.

The curves A and B correspond respectively to transmembrane pressures ΔPof 1.5 and 3 bars.

FIG. 8 is a graph representing the change in the density of the filteredflow J in kg/h.m² in relation to the time in minutes during filtrationtests on poor cod oil on an ultra-filtration membrane at 60° C.

Curves A and B correspond respectively to partial pressures of CO₂ of 0bar and of 85 bars.

The installation for the implementation of the method according to theinvention according to a method of continuous operation comprises,according to FIG. 1, means of supplying fluids for a continuous andsimultaneous feed of a viscous fluid from which one wishes to separatecertain molecular or particulate components by tangential filtration,and a third substance.

The supply means comprise for example a reservoir or tank of viscousliquid 1 and a reservoir of third substance 2 for example CO₂, each ofthese reservoirs is connected by means of a pipe 3, respectively 4,fitted with a flow meter 5, respectively 6, to a pump 7, respectively 8.The two pumps 7 and 8 are preferably dosing pumps coupled so as to keepthe proportions of the two fluids constant, even if the total flow ratesvary. The type of these pumps may be varied, they can for example bepiston pumps, membrane pumps or any other type of pump capable ofproviding a precise dose of the constituents. In the case where thethird substance supplied is in the gaseous state, it can be fed by acompressor (not shown). The supply means can also play the role of othermeans of pressurization for the whole of the fluids, or thepressurization means for these fluids may also be provided.

Pressurization is provided for example by the pumping systems describedabove or the pressurization means comprise means of introducing aneutral gas such as helium or nitrogen.

The means of introducing a neutral gas can be a high pressure supplysource, constituted by high pressure commercial bottles, or acompressor.

In the case of non-continuous operation or batch operation, the supplymeans comprise feed pumps for the viscous liquid and the third substancethat are shut down once charging has been carried out.

The single phase liquid solution, also called the “liquid phase” thatforms in the dissolution means such as the function 9 by dissolution ofthe third substance in the viscous liquid is then conditioned inconditioning means, for example it is brought to the desired temperatureby passage, for example within heating means, for example a heatexchanger 10. The dissolution means can also consist of an in-lineinjection base or a static mixer.

The installation shown in FIG. 1 additionally comprises pre-treatmentmeans or means of implementing an extra step, consisting of adecantation pot 11 provided for the case where an “anti-solvent” effectoccurs that leads to separation of heavy components. These heavycomponents are then discharged through pipe 12.

The installation for the implementation of the method according to theinvention also comprises means of transferring and circulating thesingle phase liquid solution. These means comprise, for example, a tankforming a liquid phase reservoir 13 and a pump called a recirculationpump 14 which enables the liquid phase to be passed into the tangentialfiltration means 15.

The recirculation pump 14 can be, for example a paddle pump, a pistonpump, a centrifugal pump or a gear pump. The discharge pressure of thispump must be greater than the pressure drop of the circuit including thefiltration means such as the membrane or membranes 20, that is to staytypically of the order of 1 to 10 bars depending on the residualviscosity of the liquid phase. This pump also has the purpose ofproviding a suitable speed of circulation into the tangential filtrationmeans.

The circulation means also preferably include a recirculation loop 16,fitted, for example, with a valve 17 and a flow meter 18. Thisrecirculation loop allows part of the retained material to be circulatedwithin a closed circuit from the liquid phase reservoir to thefiltration means until the desired concentration factor has beenobtained.

The filtration means 15 comprise, for example, a membrane or an assemblyof membranes 20 arranged in a housing or a tangential filtrationenclosure. The number of membranes is variable and can range, forexample, from 1 to 1000 or more.

Because the tangential filtration can be applied to the separation ofparticles or of molecules of very different sizes, the membrane(s) usedwill preferably have cutoff thresholds that are appropriate to thespecies to be separated. The diameter of the pores, for example, rangefrom about one or more micrometers that is to say 100 μm to one or morenanometers that is to say 100 μm, which thereby covers the range ofseparation given by the micro-filtration, ultra-filtration andnano-filtration membranes.

The membranes used will preferably be made of ceramic or made up ofmetal oxides such as Al₂O₃, ZrO₂, TiO₂. The membranes will preferably bemade of alumina, but membranes with a carbon support can be used andeven organic membranes such as polysulfone membranes of the “Nafion®”type, always of course with the condition that this membrane ischemically resistant to the third substance.

The installation described also includes means to lower the pressure ofthe concentrate and the permeate (depressurize) and to separate thepermeate from the filtrate and from the third substance, and to separatethe part of the retained material called the “concentrate” from theresidue and from the third substance.

These pressure reducing and separation means essentially compriseseparator “pots” of the traditional technology 21, 22 such as thosetypically used in processes that make use of a super-critical fluid.They may, for example be separators of the de-aerator, type or of thekinetic type (cyclone separator).

The separators are supplied with permeate and concentrate by pipes 23,24, fitted respectively with valves 25 and 26 and flow meters, forexample mass flow meters 27 and 28.

Only a part of the flow of retained material is discharged asconcentrate, and then separated in the concentrate separator 22. Forexample, if a concentration factor of 10 has been fixed, 10% of theretained material is discharged to provide the concentrate, the latteryielding the residue and the third substance while the remainder of theretained material will be recirculated.

According to the invention, preferably, the installation furthercomprises recycling means for the third substance stemming from theseparation from the permeate and from the concentrate, to theinstallation input, that is to say to the third substance reservoir 2.These means include the pipes 29, 30 and 31.

In the case of supply by pump, the temperature is preferably adjusted byusing heat exchange means such as a cold exchanger provided upstreamfrom the third substance reservoir, for example a liquefied gasreservoir. Hence for CO₂ at 50 bars, the temperature will be brought to10° C. using the exchanger 32.

Finally the installation comprises regulation means. In effect in acontinuous process, the concentrate and the permeate are dischargedsimultaneously in such a way that the inlet/outlet material balance isat any time in equilibrium. This is the role of the regulation of thefeed and tapping off systems. Overall, this regulation will be linked tothe flow rate of the permeate.

At a constant transmembrane pressure, the flow rate of the permeatedepends on the operating conditions chosen and also on the behavior ofthe membrane. It is known that this behavior varies, of itself, notablythrough the effect of clogging. The way the installation behaves willtherefore be dependent on fluctuations in the flow rate of the permeate.Therefore it is this flow rate which will control the input of fluids,namely the third substance and the viscous liquid, as well as the outputof the concentrate.

The regulation directly linked to the permeate is essentially thetransmembrane pressure, as in any tangential filtration process.

Furthermore, the flow rate of concentrate is detected by a mass flowmeter. For a chosen concentration factor of 10, and assuming that thesolubility of the third substance such as CO₂ does not vary between thepermeate and the concentrate, 90% of the material entering will bedischarged in the permeate and 10% in the concentrate. The flow meterinstalled at the permeate outlet then allows one to servo-operate theopening of the concentrate valve to this ratio.

In order to bring the input/output material balance into equilibrium,the feed pumps 7, 8 are servo-operated to the sum of the output flowrates, either directly through the flow meters 27, 28 or by a constantlevel in the charge pot 13 (only in the case where one is operating bypressurizing the liquid phase with a gas), or at constant pressure inthe recirculation loop when it is entirely full of liquid phase.

The fluidity of the retained material can be optimized byservo-operating the feed pump for the third substance 8 to the viscositymeasurement. The proportion of the third substance to the liquid ismodified until the viscosity level is reached. This possibility allowsone to operate at an optimum viscosity whatever changes intervene duringthe course of time, for example: changes in the concentration of theliquid phase, in the nature of the supply etc.

Once the minimum viscosity is reached, one can servo-operate the flowrate from the recirculation pump 14 to the measurement of the pressuredrop caused by drag between the inlet and the outlet of the membrane soas to work under the restriction of a constant partition. This way ofoperating has become the usual way of working in atmospheric tangentialfiltration.

The method of the invention will now be described with the help of thefollowing examples given for illustrative purposes and which are nonrestrictive.

EXAMPLES

The tangential filtration tests have been carried out using aninstallation having substantially the configuration already describedabove and adapted for operation in a non-continuous fashion or withbatches.

The boiler engineering and the piping are provided for operation at amaximum pressure of 350 bars and a temperature of 100° C.

Two types of membrane support were used; a single channel cylindricalsupport made of α-alumina manufactured by the company SCT/US Filter anda multi-channel support in the shape of a clover leaf manufactured in amixture of TiO₂/ZrO₂/Al₂O₃ and marketed by the company TAMI.

The ultra-filtration tests were carried out with:

an SCT membrane of cylindrical geometry and of pore diameter 20 nm.

a TAMI membrane of clover-leaf geometry, of pore diameter 50 nm.

The nano-filtration tests were carried out on a membrane manufactured bythe CEA from an SCT cylindrical support. The filtering layer made ofTiO₂ has a mean pore diameter of 3 nm.

The liquids studied are polyethylene glycols of molecular mass 400g/mole supplied by the company Sigma/Aldrich as well as new or usedmotor oils and finally fish oils.

Pressurization is carried out with helium supplied by the companyProdair which allows a variable transmembrane pressure to be providedwith a partial pressure of CO₂ equal to 0. In the contrary case, thepressure is given by the carbon dioxide itself supplied by the companyAir Gaz, supplied by a pump driven by compressed air which is a HaskelDSF 52 General Pneumatic pump.

The transmembrane pressure difference is detected by a Rosemount A 1151differential pressure sensor. This pressure is regulated by a Kämmer81037 needle valve controlled pneumatically. The regulation is effectedby a Eurotherm 818 regulator.

The membrane is contained in a high pressure membrane carrier situatedwithin a temperature-controlled enclosure.

The circulation pump is a Micropump Series 5000 gear pump, the gasketsof which have been modified.

The flow meter is a Deltatube pressure drop system (Midwest 300 probe).The filtrate flow rate is measured by a “balance plus chronometersystem”. The flow rate of gas is read on a gas volume counter of the“Flonic” type from the company Schlumberger.

Example 1

This example illustrates the tangential filtration of model compoundsPEG 400 (polyethylene glycol) the molar mass of which is 400 g.

The flow rates of permeate PEG and CO₂ are measured separately, inrelation to the operating conditions which are mainly the partialpressure of C₂, the temperature and the transmembrane pressure. The flowrate of the retained material is kept to a laminar condition in themembrane.

The results will be expressed as a mass flow density and a masspermeability.

For the PEG, the units are respectively kg of PEG per hour and persquare meter (kg.h⁻¹.m⁻²) and kg of PEG per hour, per square meter andper bar of transmembrane pressure (kg.h⁻¹.m⁻².bar⁻¹).

A) Filtration of PEG 400 on a Nano-filtration Membrane at 60° C.

The tests were carried out at the following partial pressures of CO₂: 0,30, 60, 90, 120, 150 bars.

The circulation of the retained material is regulated in such a way thatthe regime is a laminar one in the membrane. The transmembrane pressurevaries from 5 to 30 bars.

The results are gathered together in Table I below and are representedon the graph in FIG. 2 where the flow densities are shown as a functionof the transmembrane pressure (in bars).

The curves A, B, C, D, E, and F correspond respectively to partialpressures of CO₂ of 0, 30, 60, 90, 120, 150 bars.

B) Filtration of PEG 400 on an Ultra-filtration Membrane at 60° C.

The test conditions are the same as in paragraph A) but fortransmembrane pressures compatible with the ultra-filters, being from0.2 to 4 bars.

The results are gathered together in Table I below and are representedon the graph in FIG. 3 where the flow densities (kg/h/m²) are shown as afunction of the transmembrane pressure (in bars).

The curves A, B, C, D, E, and F correspond respectively to partialpressures of CO₂ of 0, 52, 61, 91, 121 and 151 bars.

TABLE I Comparison of the flow densities between nano- filtration andultra-filtration membranes at 60° C. Nano-filter 3 Ultra-filter PressureTrans- Flow Pressure Trans- Flow of membrane density of of membranedensity of CO₂ pressure PEG CO₂ pressure PEG P(bar) ΔP(bar) J(kg/h.m²)P(bar) ΔP(bar) J(kg/h.m²)  0  5 0.41  0 0.8 10.45 10 0.81 0.17 21.36 151.21 52 0.2 2.58 20 1.65 0.8 21.51 30 2.44 1 27.20 30 10 1.07 1.5 37.4620 1.67 1.6 44.39 30 2.37 61 0.4 20.76 60 10 1.44 0.8 31.36 20 2.63 1.239.09 30 3.61 1.6 50.00 90 10 — 2 57.73 20 3.09 2.3 61.36 30 3.88 91 0.324.55 120  10 2.74 0.4 20.83 20 3.27 0.5 27.73 30 4.33 0.8 38.64 150  102.07 1.2 42.05 20 3.13 1.6 54.55 30 4.12 121  0.3 21.25 0.5 36.36 0.840.45 1.2 75.23 1.6 84.09 151  0.5 33.25 1 47.66 1.4 59.09 2 86.29 4157.95

In both cases, ultra-filtration and nano-filtration, the maximum flow isreached at 120 bars.

For the nano-filtration membrane, the improvement factor brought aboutby dissolving the CO₂ reaches 1.7.

For the ultra-filtration membrane, the improvement factor brought aboutby dissolving the CO₂ reaches 4.2.

At 60° C., and for similar CO₂ partial pressure values, the increase inpermeability between the nano-filtration membrane and theultra-filtration membrane increases by a factor of 4.2/1.7=2.5.

C) Filtration of PEG 400 on an Ultra-filtration Membrane at 40° C.

The test conditions are the same as in paragraph B above, except withregard to the temperature. The results are gathered together in Table IIbelow and are represented on the graph in FIG. 4 where the curves A, B,C and D correspond respectively to partial pressures of CO₂ of 0, 101,122 and 152 bars.

Under these operating conditions, it appears that the maximumpermeability is reached at a partial pressure of CO₂ equal to 150 bars.Nevertheless these values are not very different from the flow densitiesat 100 and 120 bars.

The improvement bactor brought about by the presence of dissolved CO₂ isof the order of 3 against 4.2 at 60° C.

TABLE II PEG 400 flow densities for an ultra-filtration membrane at 40°C. Transmembrane Pressure of CO₂ pressure PEG flow density P(bar) ΔP(bar) J(kg/h · m²) 0 0.7 7 0.8 12 1.7 13 4 26 101 1.2 15 2.5 38 4 64 694 122 1,4 18 3 50 4.5 71 6 92 152 3 56 4 64 6 105

D) Filtration of PEG 400 on an Ultra-filtration Membrane at 75° C.

The test conditions are the same as in paragraph B above. The resultsare gathered together in Table III below and are represented on thegraph in FIG. 5 where the curves A, B, C, D and E correspondrespectively to partial pressures of CO₂ of 0, 53, 103, 122 and 153bars. The maximum permeability is reached for a partial pressure of CO₂equal to about 150 bars.

The improvement factor brought about by the dissolved CO₂ reaches amaximum of 3.1.

TABLE III PEG 400 flow densities on an ultra-filtration membrane at 75°C. Transmembrane Pressure of CO₂ pressure PEG flow density P(bar) ΔP(bar) J(kg/h · m²) 0 0.8 17 1.7 29 53 0.2 11 0.5 20 0.8 27 1.2 35 1030.4 30 0.8 51 1.6 84 3 108 122 0.5 25 1 58 2 73 3 101 4 145 153 0.4 370.8 45 1.2 72 1.6 95

From the set of results obtained with PEG 400 in Example 1, it emergesthat under the same operating conditions, the third substance plays amore favorable part for membranes with “large pores” (ultra-filtration)than for membranes with “small pores” (nano-filtration). Thisimprovement is not therefore significant solely from the viscosityeffect. In effect, in the case of nano-filtration, the pore radius ofthe membrane (≈1.5 nm) is getting close to the size of the PEG molecule.Interactions exist between the pore wall and the polymer. This is nolonger true with the ultra-filtration membrane which has a pore radiusof 10 nm. These interactions can be steric,—due to the volume taken upby the molecule—or can be surface interactions—due to physico-chemicalphenomena.

As for the effect of temperature on the permeability; if in the absenceof CO₂ the ratio of permeabilities J/ΔP (that is to say the slopes ofthe flow density lines) are practically in inverse ratio to theviscosities, in the presence of CO₂ this relationship is no longervalid.

In effect, the phenomenon of dilution by the third substance is thensuper-imposed on the phenomenon of viscosity reduction.

It is understood that there is interest in diluting the permeate flow tothe least degree possible to provide a flow of filtrate ultimately; thatis to say, the “permeate flow” minus the “third substance flow” shouldbe as high as possible.

The maximum filtrate flow will then be the result of an optimum betweenthe lowering of the viscosity and the increase in the volume dilution.This optimum is achieved in the region of 120 to 150 bars.

The maximum improvement factor obtained at different temperaturesremains roughly constant and is between 3 and 4.

Example 2

This Example illustrates tangential ultra-filtration of new or usedcommercial motor oils.

The new oil is an ESSO brand SAE 15W 40 type, with a kinematic viscosityof 40 cSt at 40° C., and a dynamic viscosity of 5.1 mPa·s at 100° C. inaccordance with the standard SAEJ 300 (16). The viscosity obtained atother temperatures can be estimated from ASTM charts. The resultsobtained are collected together in Table IV which follows:

TABLE IV Temperature ° C. 40 60 75 100 Kinematic 40 17 11 5.6 viscositycSt Dynamic 37 16 10 5.1 viscosity mPa · s

The used oil corresponds to oil that was initially a 15W40 oil drainedfrom a petrol automobile after 10,000 km of use, the automobile havingbeen driven a total distance of 80,000 km. This corresponds to an enginewith average wear.

A) Filtration of New Oil on a “TAMI” Membrane at 75° C.

The tests were carried out at the following partial pressures of CO₂: 0,51, 76, 101, 110, 120 and 151 bars.

The results are gathered together in Table V below and are representedon the graph in FIG. 6 where the flow densities J are plotted as afunction of the transmembrane pressure ΔP (in bars).

The curves A, B, C, D, E, F and G correspond to partial pressures ofCO₂: 0, 51, 76, 101, 110, 120 and 151 bars.

TABLE V Transmembrane Pressure of CO₂ Pressure PEG flow density P(bar)ΔP (bar) J(kg/h · m²) 0 0.5 1.1 1.0 6.5 3.0 15.5 4.5 14.7 6.0 13.6 511.0 13.6 2.0 20.9 76 1.0 10.9 2.0 22.0 101 0.5 17.0 1.0 22.1 1.5 36.83.0 59.1 110 1.0 20.5 120 1.0 16.5 151 1.3 26.4 2.1 36.4 5.0 60.9

B) Filtration of Used Oil on a “TAMI” Membrane at 75° C.

The operating conditions were the same as in paragraph A above but apartial pressure of CO₂ of 101 bars was used and transmembrane pressuresof from 3 to 1.5 bars were applied.

The results are gathered together in Table VI below and are representedin FIG. 6 where the line H describes the value of the stabilized flowdensity of the used oil for a CO₂ pressure equal to 101 bars. This isthe line of least slope.

TABLE VI Transmem Pressure of Brane Duration of Oil flow PermeabilityCO₂ Pressure the test density J/ΔP(kg/h · m² · P(bar) ΔP (bar) t (hours)J(kg/h · m²) bar) 100 3 0.5 16.00 5.3 1 16.00 5.3 2 11.17 3.7 3 11.823.9 4 11.27 3.8 6 10.81 3.6 7 9.29 3.1 10 8.79 2.9 14 7.86 2.6 17 7.882.6 20 9.17 3.1 1.5 25 5.95 4.0 28 5.80 3.9

The change in the permeability to the used oil over the course of timeis shown in FIG. 7.

Curves A and B correspond respectively to transmembrane pressures of ΔPof 1.5 and 3 bars.

The change in permeability over the course of time stabilizes itselfaround 2.6 kg.h⁻¹.m⁻².bar⁻¹ under a transmembrane ΔP of 3 bars butreaches 3.9 kg.h⁻¹.m⁻².bar⁻¹ under a ΔP of 1.5 bars.

Example 3

This example illustrates the tangential filtration of fish oil.

The treated fish oil is poor cod oil.

The poor cod is a fish of the gadidae family, close to the cod. The rawoil that arises from pressing the flesh, and obtained after decantation,is rich in C20 and C22 polyunsaturated fatty acids, notablyeicosapenta-enoic acid (EPA) and docosahexa-enoic acid (DHA) which areof potential interest in the prevention of cardiovascular illness. Theenrichment of the triglyceride fraction of the oil containing these longchain, fatty acids is possible through nano-filtration. The lowering ofthe viscosity is obtained, as in the previous examples by dissolution ofcarbon dioxide under pressure.

So as to study the hydrodynamic behavior of the oil and the CO₂ in aporous medium, the tests were carried out on an SCT 20 nmultra-filtration membrane.

The operating conditions were

CO₂ pressure: 85 bar

Transmembrane pressure: 5 bar

Temperature: 60° C.

The results obtained are shown on the graph in FIG. 8 where thevariation in flow density is shown as a function of time.

The curves A and B correspond respectively to partial pressures of CO₂of 0 bar and 85 bars.

After a large reduction in the flow rate of filtrate equivalent to thatobserved during the treatment of the motor oil on the SCT membrane, theintroduction of CO₂ at t=150 minutes causes an immediate effect: theflow of filtrate immediately changes from 5 kg/h.m² to a value between20 and 25 kg/h.m².

The improvement factor immediately obtained is therefore between 4 and5.

Under the operating conditions used, the proportion of CO₂ is 20%.

Example 4

This example is a comparative example with the purpose of comparing theenergy efficiency of the filtration of motor oil by the process calledthe REGELUB® process described above and by the method according to theinvention using the dissolution of a third substance.

The direct comparison between the Poiseuille formula which describes thelaminar condition and the Blasius formula which describes the turbulentcondition would give a ratio of the pressure drop due to drag of theorder of 200 times greater for the turbulent condition, that is to sayfor the Regelub process.

However this is not realistic; particularly if one takes into account,not only the pressure drops in the membrane but also those in the entirerecirculation circuit.

In this case, taking into account the various uneven parts in thepiping, it is reasonable to compare regimes that are turbulent overallusing the Blasius formula below:$\frac{\Delta \quad P}{L} = {{0.0395.U^{1.75}} \cdot \rho^{0.75} \cdot \mu^{- 0.25} \cdot D^{- 1.25}}$

where

μ represents the viscosity in Pa·s

ρ represents the density in kg/m³

U represents the speed in m/s

D represents the hydraulic diameter in m $\frac{\Delta \quad P}{L}$

represents the pressure drop per unit of length

Our tests were carried out with a clover-leaf membrane, for a mean speedof the order of 0.5 m/s to 1 m/s and a viscosity of the order of 4mPa·s, giving a mean permeability L_(P) of 4 kg.h⁻¹.m⁻².bar⁻¹.

The mean result given by the REGELUB® process, namely 13kg.h⁻¹.m⁻².bar⁻¹ has been provided with a membrane of circular sectionfor speeds of from 5 to 7 m/s with viscosities of the order of 1 mpa.s.

The operating conditions for each of the methods are given moreprecisely in Table VII below

TABLE VII Method with a third substance Regelub Process (invention) μ(mPa · s) 1 4 ρ (kg/m³) 900 900 U (m/s) 5 1 D (m) 7 × 10⁻³ 3.6 × 10⁻³ Lpoil (kg · h⁻¹ · m⁻² · bar⁻¹) 13 4

The hypothesis is also formulated that the density in the presence ofCO₂ under pressure practically does not change, as is the case for thePEGs.

Under these conditions, and with these hypotheses which are veryunfavorable for the method with the third substance according to theinvention, finally one obtains a ratio of pressure drops 10.3 timesgreater for the Regelub Process.

This figure is to be compared with the permeabilities for the oil, theratio of which is as follows:

13 kg.h⁻¹.m⁻².bar⁻¹ _((regelub))/4 kg.h⁻¹.m⁻².bar⁻¹_((third substance))≈3.25

With the hypotheses mentioned and assuming that the major part of theenergy is consumed by the drag forces, (one can also assume that theenergy for reheating to 300° C. for the Regelub Process is equivalent tothe energy for injection and for recycling the CO₂ in the method withthe third substance), comparing the energy gives a result of 10.3/3.25or 3.2 times more favorable for the filtration method which lowers theviscosity by using a third substance according to the invention.

The general conclusion from these examples is that the method and theinstallation according to the invention, for very moderate temperatures,for example of the order of 80° C. and for all the viscous liquidstreated, an improvement factor for the permeability to the filtrate ofthe order of 3 to 5 is possible in relation to filtration without athird substance at the same temperatures.

For the used motor oils, the improvement factor for the filtration at75° C. brought about by the dissolution of CO₂ is of the order of 4,while the permeability of the membrane reaches a value of 4kg.h⁻¹.m⁻².bar⁻¹ but it is obtained under conditions that are much lessdrastic than the very severe conditions of the REGELUB® Process, that isto say a temperature of 300° C. and a circulation speed of 5 to 7 m/s.Because of this, the energy efficiency of the method according to theinvention is 3.2 times more favorable and the safety and the reliabilityof the process and of the installation according to the invention areclearly better.

What is claimed is:
 1. Method of filtration of a viscous liquid, saidviscous liquid comprising at least one heavy component and at least onelight component, and having an initial viscosity under the ordinaryconditions of 0.01 Pa·s. to 1 Pa·s, said method being characterized inthat, under the effect of pressure, a super-critical substance in thesuper-critical state is dissolved in the viscous liquid, thissuper-critical substance having a viscosity lower than that of saidviscous liquid, yielding a single phase liquid solution having a lowerviscosity in comparison with the initial viscosity of the pure viscousliquid, said single phase liquid solution being treated under pressureby tangential filtration to produce a retained material comprising aportion of the super-critical substance and the heavy component, and apermeate comprising a portion of the super-critical substance and thelight components.
 2. Method according to claim 1 characterized in thatthe super-critical substance is chosen from among compounds that aregaseous under ordinary conditions of temperature and pressure and arenon-reactive to the viscous liquid.
 3. Method according to claim 2characterized in that the super-critical substance is chosen from among,carbon dioxide, helium, nitrogen, nitrogen monoxide, sulfurhexafluoride, gaseous alkanes with from 1 to 5 carbon atoms, gaseousalkenes having from 2 to 4 carbon atoms, gaseous alkynes having from 2to 4 carbon atoms, gaseous dienes, gaseous chlorinated and/orfluorinated hydrocarbons, and their mixtures.
 4. Method according toclaim 1 characterized in that the super-critical substance is chosenfrom among compounds that are liquid under ordinary conditions oftemperature and pressure and are non-reactive to the viscuous liquid. 5.Method according to claim 4 characterized in that the super-criticalsubstance is chosen from among liquid alkanes with from 5 to 20 carbonatoms, liquid alkenes with from 5 to 20 carbon atoms, liquid alkyneswith from 4 to 20 carbon atoms, alcohols, ketones, ethers, esters,liquid chlorinated and/or fluorinated hydrocarbons or their mixtures. 6.Method according to claim 1 characterized in that the single phaseliquid solution has a viscosity of about from one tenth to about onehundredth of the initial viscosity of the pure viscous liquid.
 7. Methodaccording to claim 1 characterized in that the temperature of filtrationis from 20 to 200° C.
 8. Method according to claim 1 characterized inthat said viscuous liquid is circulated a at the speed of 0.5 to 10 m/s.9. Method according to claim 8 characterized in that the speed ofcirculation is from 1 to 5 m/s.
 10. Method according to claim 1characterized in that the working pressure is from 30 to 500 bars. 11.Method according to claim 10 characterized in that the working pressureis from 50 to 300 bars.
 12. Method according to claim 1 characterized inthat the pressure is provided by the pressure of an excess of thesuper-critical substance added to said viscous liquid in a gaseous orsupercritical state.
 13. Method according to claim 1 characterized inthat the pressure is provided by adding a neutral gas other than thethird substance.
 14. Method according to claim 1 additionallycharacterized in that said concentrate, and the permeate, are treated bylowering the pressure, such that the permeate is separated into afiltrate comprising light components and into a portion of thesuper-critical substance, and the concentrate is separated into aresidue comprising heavy components and into a portion of thesuper-critical substance.
 15. Method according to claim 14 characterizedin that said lowering of the pressure is carried out in several steps.16. Method according to claim 14 characterized in that thesuper-critical substance arising from the separation treatments of thepermeate and the concentrate is recycled to the method.
 17. Methodaccording to claim 1 characterized in that the super-critical substanceis in excess in relation to the single phase liquid solution.
 18. Methodaccording to claim 1 characterized in that the tangential filtration isa micro-filtration or an ultra-filtration or a nano-filtration. 19.Method according to claim 1 characterized in that the viscous liquid ischosen from among heat sensitive fluids or liquids or fluids or liquidscontaining heat sensitive products, mineral oils, industrial processoils and fluids, motor oils, used oils, fluids charged with particlesand/or heavy compounds, or petroleum oils.
 20. Method according to claim19 characterized in that the said heat sensitive fluids or liquids arechosen from among animal oils, vegetable oils, body fluids, foodproducts, liquids arising from agriculture and aqueous phases containingproteins.
 21. Method according to claim 1 characterized in that saidmethod is carried out in a non-continuous manner.
 22. Method accordingto claim 1 characterized in that said method is carried out in batches.23. Method according to claim 1, wherein said method is applied to thetreatment of used oils with a view to recycling them.
 24. Methodaccording to claim 1, wherein said method is applied to the separationof catalyst fines and/or asphaltenes from a petroleum fraction in acatalytic cracking process.
 25. Method according to claim 1, whereinsaid method is applied to the enrichment of the triglyceride fraction ofa fish oil with C20 and C22 polyunsaturated fatty acids.
 26. Methodaccording to claim 1, wherein said method is applied to the treatment ofaqueous phases containing proteins.